JP4403244B2 - Method for producing positive electrode material for lithium battery, and lithium battery - Google Patents

Method for producing positive electrode material for lithium battery, and lithium battery Download PDF

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JP4403244B2
JP4403244B2 JP2004544984A JP2004544984A JP4403244B2 JP 4403244 B2 JP4403244 B2 JP 4403244B2 JP 2004544984 A JP2004544984 A JP 2004544984A JP 2004544984 A JP2004544984 A JP 2004544984A JP 4403244 B2 JP4403244 B2 JP 4403244B2
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positive electrode
electrode material
firing
lithium battery
lithium
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重人 岡田
準一 山木
亦可 陳
貴文 山元
直樹 八田
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Kyushu University NUC
Mitsui Engineering and Shipbuilding Co Ltd
Mitsui E&S Holdings Co Ltd
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    • H—ELECTRICITY
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    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
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    • H01M4/04—Processes of manufacture in general
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    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
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    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
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Description

本発明は、リチウム電池用正極材料の製造方法、およびその正極材料を構成要素とするリチウム電池（１次電池または２次電池）に関し、より詳しくは、金属リチウム等のアルカリ金属やその合金および化合物等を負極活物質に有する金属リチウム電池、リチウムイオン電池、リチウムポリマー電池等の１次電池や２次電池に用いられる正極材料（ＦｅＰＯ４）の製造方法、およびその方法によって製造される正極材料を有するリチウム１次電池またはリチウム２次電池に関する。The present invention relates to a method for producing a positive electrode material for a lithium battery, and a lithium battery (primary battery or secondary battery) having the positive electrode material as a constituent element, and more specifically, an alkali metal such as metallic lithium, an alloy thereof, and a compound thereof. A method for producing a positive electrode material (FePO 4 ) used for a primary battery or a secondary battery such as a metal lithium battery, a lithium ion battery, or a lithium polymer battery having a negative electrode active material, and a positive electrode material produced by the method. The present invention relates to a lithium primary battery or a lithium secondary battery.

金属リチウム等のアルカリ金属やその合金および化合物等を負極活物質に有する金属リチウム電池、リチウムイオン電池、リチウムポリマー電池等の１次電池や２次電池は、容量が大きく近年脚光をあびている。これらの１次電池や２次電池に用いられる正極材料は、放電および充電の過程でリチウム等のアルカリ金属イオンのドープ／脱ドープを伴い電極酸化還元反応が進行する。この正極材料として、三方晶Ｐ３２１の結晶構造を有するリン酸第二鉄（ＦｅＰＯ４）が知られている（特許第３１２６００７号公報）。
上記特許第３１２６００７号には、リン酸第二鉄水和物（ＦｅＰＯ４・ｎＨ２Ｏ）を熱処理することによって無水リン酸第二鉄を得る合成法が記載されているが、このリン酸第二鉄水和物の合成法については示されていない。
また、Ｐ３２１三方晶のＦｅＰＯ４正極活物質としては、ＮＨ４Ｈ２ＰＯ４およびＦｅ（ＮＨ４）２（ＳＯ４）２・６Ｈ２Ｏを出発原料とした６５０℃での合成例が報告されているが［Ｐ．Ｐ．Ｐｒｏｓｉｎｉら、Ｊ．Ｅｌｅｃｔｒｏｃｈｅｍ．Ｓｏｃ．，１４０，Ａ２９７（２００２）］、その容量は僅か４０ｍＡｈ／ｇに過ぎない。
従来、焼成前駆体となるリン酸第二鉄水和物は、硫酸第二鉄や塩化第二鉄等（水和物を含む）の３価鉄含有溶液に、リン酸水素二ナトリウム等のリン酸イオン含有アルカリ性化合物を混合し、加温保持後に沈殿としてろ別する等によって合成されていたが、ナトリウムイオンなどの不揮発性元素が不純物として混入しやすいため、２次電池用の正極材料としては適していない。すなわち、この合成方法においては、ろ過工程によりナトリウムイオン等を焼成前駆体から除去する必要があり、その作業が煩雑であるとともに、不純物混入の要因となる。また、ろ過を完全に行い純度を上げるためには、沈殿として得られるリン酸第二鉄水和物の結晶を十分成長させ、大粒径（例えば、１０μｍ程度以上）とすることが好ましいが、一般に大粒径のリン酸第二鉄水和物を焼成すると、得られるリン酸第二鉄も大粒径となるため正極として活性が低くなるという問題がある。電極材料は、その粒径、粒子形状、比表面積、不純物等が電極としての性能に大きな影響を及ぼすことが知られている。
本発明の目的は、リチウム電池、すなわち、リチウム１次電池やリチウム２次電池の正極材料に適したリン酸第二鉄を確実、かつ、容易に合成可能な正極材料の製造方法、およびこの方法により得られる正極材料を有する高性能なリチウム電池（１次電池または２次電池）を提供することにある。Primary batteries and secondary batteries such as metal lithium batteries, lithium ion batteries, and lithium polymer batteries having an alkali metal such as metallic lithium or an alloy or compound thereof as a negative electrode active material have been attracting attention in recent years because of their large capacity. The positive electrode materials used in these primary batteries and secondary batteries undergo an electrode redox reaction with doping / dedoping of alkali metal ions such as lithium in the course of discharging and charging. As this positive electrode material, ferric phosphate (FePO 4 ) having a crystal structure of trigonal P 321 is known (Japanese Patent No. 3126007).
The above patent No. 3126007 describes a synthesis method for obtaining ferric phosphate anhydrous by heat-treating ferric phosphate hydrate (FePO 4 .nH 2 O). No method for the synthesis of diiron hydrate is shown.
In addition, as a P 321 trigonal FePO 4 positive electrode active material, a synthesis example at 650 ° C. using NH 4 H 2 PO 4 and Fe (NH 4 ) 2 (SO 4 ) 2 .6H 2 O as starting materials is reported. [P. P. Prosini et al. Electrochem. Soc. 140, A297 (2002)], and its capacity is only 40 mAh / g.
Conventionally, ferric phosphate hydrate, which is a calcined precursor, is added to a trivalent iron-containing solution of ferric sulfate, ferric chloride, etc. (including hydrate), and phosphorous such as disodium hydrogen phosphate. It was synthesized by mixing acid ion-containing alkaline compounds and filtering them as a precipitate after heating, but as a positive electrode material for secondary batteries because non-volatile elements such as sodium ions are likely to be mixed as impurities. Not suitable. That is, in this synthesis method, it is necessary to remove sodium ions and the like from the calcined precursor by a filtration step, which is complicated and causes contamination by impurities. Further, in order to completely perform filtration and increase the purity, it is preferable to sufficiently grow crystals of ferric phosphate hydrate obtained as a precipitate to obtain a large particle size (for example, about 10 μm or more) In general, when ferric phosphate hydrate having a large particle size is fired, the obtained ferric phosphate also has a large particle size, which causes a problem that the activity as a positive electrode is lowered. It is known that the electrode material has a large influence on the performance as an electrode due to its particle size, particle shape, specific surface area, impurities, and the like.
An object of the present invention is to produce a lithium battery, that is, a positive electrode material capable of reliably and easily synthesizing ferric phosphate suitable for a positive electrode material of a lithium primary battery or a lithium secondary battery, and this method. It is to provide a high-performance lithium battery (primary battery or secondary battery) having a positive electrode material obtained by the above.

上記課題を解決するため、本発明の第１の態様は、溶液中でリン酸イオンを遊離する化合物と、金属鉄とを混合し、前記金属鉄を溶解させて反応させた後、焼成を行うことによりリン酸第二鉄を合成することを特徴とする、リチウム電池用正極材料の製造方法である。
このリチウム電池用正極材料の製造方法によれば、正極材料（すなわち、正極活物質としてのリン酸第二鉄）を化学量論的組成比の原料から確実に、かつ容易に合成することができる。また、「溶液中でリン酸イオンを遊離する化合物」と金属鉄との反応過程は水溶液中で行えるので取扱いが容易である。また、類縁の正極材料として知られるオリビン型（結晶構造Ｐｎｍａ）リン酸鉄（ＩＩ）リチウム等と異なり、本正極材料では焼成過程において鉄を＋３価まで酸化させることから空気存在下で行うことができるので、例えば水素等を用いて還元雰囲気下とするなどの特別な条件を設定することを要さず簡易である。
本発明の第２の態様は、溶液中でリン酸イオンを遊離する化合物と、金属鉄とを水溶液中で擂潰しながら反応させた後、焼成を行うことによりリン酸第二鉄を合成することを特徴とする、リチウム電池用正極材料の製造方法である。
このリチウム電池用正極材料の製造方法によれば、正極材料（正極活物質）としてのリン酸第二鉄を化学量論的組成比の原料から確実に、かつ容易に合成することができる。また、「溶液中でリン酸イオンを遊離する化合物」と金属鉄との反応過程は水溶液中で行えるので取扱いが容易である。また、原料混合物を擂潰しながら反応させることにより、反応を促進させることができる。
また、類縁の正極材料として知られるオリビン型リン酸鉄（ＩＩ）リチウム等と異なり、本正極材料では焼成過程において鉄を＋３価まで酸化させることから空気存在下で行うことができるので、例えば水素等を用いて還元雰囲気下とするなどの特別な条件を設定することを要さず簡易である。
また、本発明の第３の態様は、前記第１または第２の態様において、前記溶液中でリン酸イオンを遊離する化合物が、リン酸、五酸化リン、またはリン酸二水素アンモニウムであることを特徴とする、リチウム電池用正極材料の製造方法である。
このリチウム電池用正極材料の製造方法によれば、第１または第２の態様の作用効果に加え、原料にナトリウムなどの不揮発性元素を含まないため焼成過程において不純物を除去することができ、不純物をほとんど含有しないリン酸第二鉄を化学量論的な仕込み組成の原料混合物から合成することができる。従って、この方法により製造されるリン酸第二鉄はリチウム電池用の正極材料として好適に使用することができる。また、これらの原料はリン酸及び鉄の最も基本的な１次原料またはこれに類する原料であるので比較的安価であり、また極めて高純度のものが容易に入手でき、しかも取扱いが容易であるため、量産性に優れている。
また、本発明の第４の態様は、第１から第３のいずれか１つの態様により製造された正極材料に導電性炭素を混合し、粉砕混合することを特徴とする、リチウム電池用正極材料の製造方法である。この第４の態様によれば、正極材料であるリン酸第二鉄の粒子表面に炭素を被覆複合化することにより、この正極材料を用いるリチウム電池の放電容量を大幅に改善し、性能向上を図ることができる。
また、本発明の第５の態様は、前記第１から第４のいずれかの態様により製造された正極材料を構成要素とすることを特徴とする、リチウム電池である。本発明方法によって製造された正極材料を有するリチウム電池は、正極材料の電気化学的特性が優れているために、リチウム電池として高性能な電池特性を発現することができる。In order to solve the above-mentioned problems, the first aspect of the present invention is a method in which a compound that releases phosphate ions in a solution and metallic iron are mixed, the metallic iron is dissolved and reacted, and then fired. This is a method for producing a positive electrode material for a lithium battery, characterized in that ferric phosphate is synthesized.
According to this method for producing a positive electrode material for a lithium battery, a positive electrode material (that is, ferric phosphate as a positive electrode active material) can be reliably and easily synthesized from a raw material having a stoichiometric composition ratio. . Further, the reaction process of “compound that liberates phosphate ions in solution” and metallic iron can be carried out in an aqueous solution, so that it is easy to handle. Also, unlike the olivine type (crystal structure Pnma) lithium iron (II) phosphate known as a related positive electrode material, this positive electrode material is oxidized in the presence of air because iron is oxidized to +3 in the firing process. Therefore, it is simple without requiring special conditions such as a reduction atmosphere using hydrogen or the like.
A second aspect of the present invention is to synthesize ferric phosphate by reacting a compound that liberates phosphate ions in solution with metallic iron in an aqueous solution while crushing, and then firing. This is a method for producing a positive electrode material for a lithium battery.
According to this method for producing a positive electrode material for a lithium battery, ferric phosphate as a positive electrode material (positive electrode active material) can be reliably and easily synthesized from a raw material having a stoichiometric composition ratio. Further, the reaction process of “compound that liberates phosphate ions in solution” and metallic iron can be carried out in an aqueous solution, so that it is easy to handle. Moreover, reaction can be accelerated | stimulated by making it react, crushing a raw material mixture.
Further, unlike olivine type lithium iron (II) phosphate known as a related positive electrode material, since this positive electrode material oxidizes iron to +3 in the firing process, it can be performed in the presence of air. It is simple without setting special conditions such as using a reducing atmosphere.
In addition, according to a third aspect of the present invention, in the first or second aspect, the compound that liberates phosphate ions in the solution is phosphoric acid, phosphorus pentoxide, or ammonium dihydrogen phosphate. This is a method for producing a positive electrode material for a lithium battery.
According to this method for producing a positive electrode material for a lithium battery, in addition to the effects of the first or second aspect, impurities can be removed during the firing process because the raw material does not contain a non-volatile element such as sodium. Can be synthesized from a raw material mixture having a stoichiometric charge composition. Therefore, ferric phosphate produced by this method can be suitably used as a positive electrode material for a lithium battery. In addition, these raw materials are the most basic primary raw materials of phosphoric acid and iron or similar raw materials, so that they are relatively inexpensive and can be easily obtained in extremely high purity and easy to handle. Therefore, it is excellent in mass productivity.
According to a fourth aspect of the present invention, there is provided a positive electrode material for a lithium battery, characterized in that conductive carbon is mixed and pulverized and mixed with the positive electrode material manufactured according to any one of the first to third aspects. It is a manufacturing method. According to the fourth aspect, the surface of the ferric phosphate particles, which are the positive electrode material, is coated and composited with carbon, thereby greatly improving the discharge capacity of the lithium battery using this positive electrode material and improving the performance. Can be planned.
According to a fifth aspect of the present invention, there is provided a lithium battery characterized in that the positive electrode material produced by any one of the first to fourth aspects is a constituent element. A lithium battery having a positive electrode material manufactured by the method of the present invention can exhibit high-performance battery characteristics as a lithium battery because the electrochemical characteristics of the positive electrode material are excellent.

図１は、実施例１で得た正極材料のＸ線回析結果図面である。
図２は、実施例１で得た２次電池の充放電特性を示すグラフ図面である。
図３は、実施例１で得た２次電池の充放電特性を示すグラフ図面である。
図４は、実施例１で得た２次電池の充放電特性を示すグラフ図面である。
図５は、実施例２で得た正極材料のＸ線回析結果を示す図面である。
図６は、実施例３で得た正極材料のＸ線回析結果を示す図面である。
図７は、実施例４における各温度で焼成された正極材料のＸ線回析結果を示す図面である。
図８は、実施例４における各温度で焼成された正極材料を使用した２次電池の充放電特性を示すグラフ図面である。
図９は、実施例５で得た２次電池の充放電特性を示すグラフ図面である。
図１０は、実施例６で得た正極材料のＸ線回析結果を示す図面である。1 is an X-ray diffraction pattern of the positive electrode material obtained in Example 1. FIG.
FIG. 2 is a graph showing the charge / discharge characteristics of the secondary battery obtained in Example 1.
FIG. 3 is a graph showing the charge / discharge characteristics of the secondary battery obtained in Example 1.
FIG. 4 is a graph showing the charge / discharge characteristics of the secondary battery obtained in Example 1.
FIG. 5 is a drawing showing the results of X-ray diffraction of the positive electrode material obtained in Example 2.
6 is a drawing showing the results of X-ray diffraction of the positive electrode material obtained in Example 3. FIG.
FIG. 7 is a drawing showing the results of X-ray diffraction of the positive electrode material fired at various temperatures in Example 4.
FIG. 8 is a graph showing charge / discharge characteristics of a secondary battery using a positive electrode material fired at each temperature in Example 4.
FIG. 9 is a graph showing the charge / discharge characteristics of the secondary battery obtained in Example 5.
FIG. 10 is a drawing showing the results of X-ray diffraction of the positive electrode material obtained in Example 6.

本発明のリチウム電池用正極材料の製造方法は、溶液中でリン酸イオンを遊離する化合物と金属鉄を水溶液中で擂潰しながら反応させた後、焼成を行うことによって実施される。
＜正極材料＞
本発明方法によって得られる正極材料は、一般式ＦｅＰＯ４で示されるリン酸第二鉄である。リン酸第二鉄は、原料を反応させたのち、反応生成物を空気存在下（酸化雰囲気下）において焼成することによって合成されるものであり、結晶点群Ｐ３２１三方晶の構造をとり、リチウムイオン等の負極金属イオンの挿入・脱離反応によって繰り返し充放電可能なリチウム電池用正極材料として用いることができる。
本発明の正極材料であるリン酸第二鉄の原料としては、溶液中でリン酸イオンを遊離する化合物および金属鉄が用いられる。これらの原料は、Ｐ：Ｆｅのモル比が１：１となるように、化学量論組成比に沿って調整することが好ましい。
「溶液中でリン酸イオンを遊離する化合物」としては特に限定されないが、リン酸（Ｈ３ＰＯ４）、五酸化リン（Ｐ２Ｏ５）、リン酸二水素アンモニウム（ＮＨ４Ｈ２ＰＯ４）、リン酸水素ニアンモニウム［（ＮＨ４）２ＨＰＯ４］等を挙げることができる。この中では、鉄を溶解する過程で比較的強い酸性条件を保つことが好ましいため、リン酸、五酸化リン、リン酸二水素アンモニウムが好ましい。
原料としてリン酸を用いる場合には、通常リン酸は水溶液の状態で市販されているために、その含有率（純度）を滴定等により正確に求めた後に使用することが好ましい。金属鉄は、反応を促進させるため微粉末状態（粒径２００μｍ以下、より好ましくは１５０μｍ以下、望ましくは１００μｍ以下）であるものが好ましい。
このように、本発明では、金属鉄のような１次原料またはこれに類する原料を仕込むことによって、容易にリチウム電池用正極材料としてのリン酸第二鉄を得ることができる。また、ナトリウム等の不揮発性元素が原料に含まれていないためにろ過などの煩雑な工程を経る必要がなく、単に焼成することによって不純物が完全に除去されるので、不純物をほとんど含まないリン酸第二鉄を合成することができる。
「溶液中でリン酸イオンを遊離する化合物」と、金属鉄との反応は、例えば「溶液中でリン酸イオンを遊離する化合物」に必要に応じて水を加えた水溶液に、金属鉄を添加することによって行うことができる。この反応に際しては、まず金属鉄を十分に溶解させることが重要である。金属鉄を溶解させるための操作として、例えば、擂潰および／または加熱（還流など）を行うことができる。
すなわち、「溶液中でリン酸イオンを遊離する化合物」と、金属鉄との反応過程は、原料混合物を例えば自動擂潰機、ボールミルやビーズミル等を用いて充分に混合・擂潰する操作、および／または、原料混合物を還流などの手段で加熱する操作によって実施することが好ましい。
原料混合物を擂潰することにより、金属鉄にせん断力が付与され表面が更新されて、反応を促進させることができる。擂潰に際しては、水素が発生するため適宜これを除去しながら擂潰を行うことが好ましい。最終的に反応が完了した後、これを乾燥させると極めて微細（約１μｍ以下）のリン酸第二鉄水和物が得られ、これを焼成することにより微細で高活性なリン酸第二鉄正極材料が得られる。さらに、反応を完結させるには、超音波照射を行うこともできる。また、擂潰過程は初期の水素発生が弱まり、反応が遅くなった後は空気存在下または酸素を添加した酸化雰囲気下で行い、これらにより水素を追い出しながら反応させることが好ましい。
また、加熱操作により、金属鉄の溶解反応が促進されるので、正極材料の収率を向上させ得る。還流などによる加熱は、鉄の酸化を促す目的で、空気中で行うことが好ましい。還流操作では、比較的大型化が困難な機械的微粉砕操作が不要になるため、大量生産を行う上で特に有利であると考えられる。
また、反応過程において、過酸化水素、酸素、臭素・塩素などのハロゲン、さらし粉・次亜塩素酸などのハロゲン酸化物等からなる揮発性酸化剤、またはシュウ酸、塩酸等からなる揮発性酸等の反応促進剤を存在させることによって、溶液中でリン酸イオンを遊離する化合物と、金属鉄との反応が促進され、反応を短時間で完結させることができる場合がある。ただし、酸素や酸化剤を添加する場合、発火等の危険性があるため、防爆、気相の防爆限界組成管理に留意することが好ましい。
ここで酸化剤の添加は、金属鉄を溶液中でリン酸イオンを遊離する化合物と反応・溶解（通常は２価鉄イオンとして溶解）させるだけでなく、生じた２価鉄イオンをさらに３価鉄イオンまで変化させる効果があり、後の焼成過程で生成するリン酸第二鉄正極材料中の２価鉄の残留を防止できる。また、塩酸、シュウ酸等の酸性反応促進剤は、鉄の水素発生溶解反応を促進させる効果を有する。さらに、硝酸等の揮発性で酸化力及び酸性を兼ね備える反応促進剤を加えることも効果的である。なお、これらの酸化剤・反応促進剤は焼成過程において取り除かれるため、正極材料に残存する虞はない。
溶液中でリン酸イオンを遊離する化合物と、金属鉄との反応によって生成される反応生成物を焼成することによって、正極材料としてのリン酸第二鉄が製造される。焼成は、一般に採用されるような１００〜９００℃に至る焼成過程において、適切な温度範囲および処理時間等の焼成条件を選択して実施することができる。この焼成は、鉄の酸化を促すために空気などの酸素存在下である酸化雰囲気下で行うことが好ましい。
焼成は、例えば、常温〜焼成完了温度（例えば１００〜９００℃程度、正極材料中の水分除去の観点からは、好ましくは５００〜７００℃程度、より好ましくは６５０℃程度）までの一段階の昇温及び保持過程にて行うことができる。
原料から１００〜５００℃程度の低温で焼成を行う場合（以下、「低温焼成」と記す）には、５００℃までの焼成温度ではリン酸第二鉄の大部分が非晶質である。一方、６００℃〜９００℃程度の高温で焼成を行う場合（以下、「高温焼成」と記す）においては、リン酸第二鉄の大部分が結晶点群Ｐ３２１構造をとるようになる。焼成温度が５００〜６００℃の範囲（以下、「中温焼成」と記す）は、前記非晶質から結晶点群Ｐ３２１構造に移行する温度域であり、焼成温度の上昇に伴い非晶質の比率が減少し、徐々に結晶点群Ｐ３２１構造のリン酸第二鉄が増加していき、焼成温度が６００℃を超えると、前記したように結晶点群Ｐ３２１構造が優勢になる。
低温焼成によって得られる非晶質のリン酸第二鉄、高温焼成によって得られる結晶点群Ｐ３２１構造のリン酸第二鉄、さらに中温焼成によって得られる、非晶質と結晶点群Ｐ３２１構造が混在するリン酸第二鉄について、電気化学的特性を比較すると、後記実施例４に示すように、いずれも比較的良好な放電特性を示すことが確認されている。低温焼成や中温焼成のリン酸第二鉄中には、非晶質が存在するにもかかわらず、高温焼成の放電曲線とほぼ近似した軌跡を示すことから、局所的微細構造としては結晶点群Ｐ３２１構造と同様の構造を有していることが示唆される。
従って、本発明では、低温焼成、中温焼成または高温焼成のいずれを選択してもよく、あるいは、低中温（例えば１００〜６００℃）または中高温（例えば５００〜９００℃）の温度範囲で目的とするリン酸第二鉄に応じた焼成温度を設定できる。なお、後にリチウム電池等の非水系電解質電池に適用する場合は、正極材料中には水分の残留がないことが望ましく、水分を完全に除去する観点からは高温焼成が好ましい。
また、焼成は１段階焼成に限るものではなく、低温域での焼成過程（常温〜３００ないし４００℃の温度範囲；以下、「仮焼成」と記すときがある。）と、高温域での焼成過程［常温〜焼成完了温度（５００〜８００℃程度、好ましくは５００〜７００℃程度、より好ましくは６５０℃程度）；以下、「本焼成」と記すときがある。］の２段階に分けて行うこともできる。係る場合においては、溶液中でリン酸イオンを遊離する化合物と、金属鉄との反応生成物を仮焼成し、本焼成では焼成前駆体を上記温度域において５〜２４時間程度維持することが好ましい。なお、反応生成物は必要に応じて仮焼成を行う前に乾燥、粉砕することができ、また、焼成前駆体は必要に応じて本焼成を行う前に粉砕・擂潰することができる。また、一段階による焼成及び仮焼成と本焼成とを含めて単に「焼成」と記すときがある。
以上述べたように、本発明方法によれば、焼成前駆体（リン酸第二鉄水和物）の合成において煩雑なろ過工程が不要であり、かつ、原料の焼成後に不純物が残留する虞が無く、不純物をほとんど含まない正極材料を確実に合成できる。また、原料は１次原料またはこれに類する原料であり、取扱いが容易であり、しかも安価あるため、量産化に適している。
また、上記のようにして得られた正極材料に導電性炭素を混合し、例えばボールミルなどを用いて１２時間〜３６時間程度粉砕混合を行うことによって、正極材料粒子表面に炭素を被覆させた炭素複合化正極材料を得ることができる。導電性炭素としては、例えばアセチレンブラックなどに代表されるカーボンブラック等が挙げられる。
ＦｅＰＯ４正極材料に炭素を複合化することにより、後記実施例５に示すように、炭素を複合化しない場合に比べて放電容量が大幅に改善される。つまり、炭素複合化によって正極活物質としてのＦｅＰＯ４正極材料の表面導電性を改善することが可能になり、正極活物質の利用率が格段に向上する。従って、炭素複合化は、適切に適用されるならばＦｅＰＯ４を正極材料として用いるリチウム電池の性能向上に有効である。
＜リチウム電池＞
以上のようにして得られる本発明に係る正極材料を使用したリチウム電池としては、例えば、金属リチウム電池、リチウムイオン電池、リチウムポリマー電池等の２次電池を挙げることができる。また、本発明のリチウム電池は、放電のみを行う１次電池としても用いられる。
ここで、負極材料に金属リチウムを使用した金属リチウム電池を例にとり、その基本構成を説明する。金属リチウム電池は、充放電に伴い負極の金属リチウムの電解質中への溶解と負極への析出により、リチウムイオンが正極、負極間を往復することを特徴とする、２次電池である。
負極材料としては、金属リチウム電池に用いられる金属リチウムの他に、リチウムアルミニウム合金等のリチウム含有合金、リチウムチタン複合酸化物（Ｌｉ［Ｌｉ４／３Ｔｉ５／３Ｏ４］等）、リチウム遷移金属複合窒化物（Ｌｉ７ＭｎＮ４，Ｌｉ３ＦｅＮ２等）などからなる、リチウムを初期状態から含み、中心元素が還元態である化合物を使用することができる。
電解質としては、例えば、溶媒としてエチレンカーボネート、プロピレンカーボネート、ブチレンカーボネート、γ−ブチロラクトン等の環状有機溶媒と、ジメチルカーボネート、エチルメチルカーボネート、ジエチルカーボネート、ジメトキシエタン等の鎖状有機溶媒とを混合した混合溶媒に、溶質としてＬｉＰＦ６，ＬｉＢＦ４，ＬｉＣｌＯ４，ＬｉＣＦ３ＳＯ３，ＬｉＮ（ＣＦ３ＳＯ２）２，ＬｉＣ（ＣＦ３ＳＯ２）３等の電解質物質を溶解させた液状電解質、これらの液状電解質をポリエチレンオキシド、ポリプロピレンオキシド、ポリアクリロニトリル、ポリビニリデンフルオライド等の高分子ゲル物質とともに存在させたゲルポリマー電解質、これらのゲルポリマー電解質を化学架橋した架橋ポリマー電解質等が用いられる。負極材料が金属リチウムである場合には、充電時に析出するデンドライトの生長を抑制可能なゲルポリマー電解質または架橋ポリマー電解質が好ましい。電解質が液状電解質である場合には、正極と負極との短絡を防止するためにポリエチレン・ポリプロピレンなどのポリオレフィン製等のセパレータをそれらの間に配置することにより、正極と負極とを隔離することが好ましい。
正極および負極は、その効果を損なわない範囲で正極材料および負極材料にカーボンブラック等の導電性付与剤を存在させ、また、ポリテトラフルオロエチレン、ポリ弗化ビニリデン等のフッ素系ポリマー、ポリイミド、ポリオレフィン等の結着剤、さらに必要な場合には極性有機液体を加えた混合物を混合・混練した後、シート状に成形したものを用いることができる。そして、金属泊や金属網等で集電して電池が構成される。なお、負極に金属リチウムを使用した場合には、充放電によりＬｉ（０）／Ｌｉ＋の変化が起こり、電池が形成される。
上記方法によって製造された本発明の２次電池は、正極材料の電気化学的特性が優れているために、高性能な電池特性を発現できる２次電池である。特に、負極に金属リチウムを用いた金属リチウム電池に対して使用した場合に、良好な電池特性を得ることが可能である。
次に、実施例により本発明を更に詳細に説明するが、本発明はこれらによって制約されるものではない。The method for producing a positive electrode material for a lithium battery of the present invention is carried out by reacting a compound that liberates phosphate ions and metallic iron in a solution while being crushed in an aqueous solution, followed by firing.
<Positive electrode material>
The positive electrode material obtained by the method of the present invention is ferric phosphate represented by the general formula FePO 4 . Ferric phosphate is synthesized by reacting raw materials and then firing the reaction product in the presence of air (oxidizing atmosphere), and has a structure of crystal point group P 321 trigonal crystal, It can be used as a positive electrode material for lithium batteries that can be repeatedly charged and discharged by insertion / extraction reactions of negative electrode metal ions such as lithium ions.
As a raw material for ferric phosphate, which is a positive electrode material of the present invention, a compound that releases phosphate ions in solution and metallic iron are used. These raw materials are preferably adjusted along the stoichiometric composition ratio so that the molar ratio of P: Fe is 1: 1.
The “compound that liberates phosphate ions in the solution” is not particularly limited, but phosphoric acid (H 3 PO 4 ), phosphorus pentoxide (P 2 O 5 ), ammonium dihydrogen phosphate (NH 4 H 2 PO 4). ), Diammonium hydrogen phosphate [(NH 4 ) 2 HPO 4 ] and the like. Among these, phosphoric acid, phosphorous pentoxide, and ammonium dihydrogen phosphate are preferred because it is preferable to maintain relatively strong acidic conditions in the process of dissolving iron.
When phosphoric acid is used as a raw material, since phosphoric acid is usually marketed in the form of an aqueous solution, it is preferably used after its content (purity) is accurately determined by titration or the like. The metal iron is preferably in a fine powder state (particle size 200 μm or less, more preferably 150 μm or less, desirably 100 μm or less) in order to promote the reaction.
Thus, in the present invention, ferric phosphate as a positive electrode material for a lithium battery can be easily obtained by charging a primary raw material such as metallic iron or a similar raw material. In addition, since non-volatile elements such as sodium are not contained in the raw material, it is not necessary to go through complicated steps such as filtration, and the impurities are completely removed by simply firing, so that phosphoric acid containing almost no impurities Ferric iron can be synthesized.
The reaction between “compounds that release phosphate ions in solution” and metallic iron can be achieved, for example, by adding metallic iron to an aqueous solution in which water is added as necessary to “compounds that release phosphate ions in solution”. Can be done. In this reaction, it is important to first sufficiently dissolve metallic iron. As an operation for dissolving metallic iron, for example, crushing and / or heating (such as reflux) can be performed.
That is, the reaction process of “compound that liberates phosphate ions in solution” and metallic iron is performed by sufficiently mixing and crushing the raw material mixture using, for example, an automatic crusher, a ball mill or a bead mill, and the like. It is preferable to carry out the operation by heating the raw material mixture by means such as reflux.
By crushing the raw material mixture, a shearing force is applied to the metallic iron, the surface is renewed, and the reaction can be promoted. In crushing, since hydrogen is generated, it is preferable to carry out crushing while removing it appropriately. When the reaction is finally completed, it is dried to obtain extremely fine (less than about 1 μm) ferric phosphate hydrate. By firing this, fine and highly active ferric phosphate is obtained. A positive electrode material is obtained. Furthermore, in order to complete the reaction, ultrasonic irradiation can be performed. The crushing process is preferably carried out in the presence of air or in an oxidizing atmosphere to which oxygen has been added after the initial hydrogen generation has weakened and the reaction has slowed, and the reaction is preferably carried out while expelling hydrogen.
Moreover, since the dissolution reaction of metallic iron is promoted by the heating operation, the yield of the positive electrode material can be improved. Heating by refluxing or the like is preferably performed in air for the purpose of promoting oxidation of iron. The reflux operation eliminates the need for a mechanical pulverization operation that is relatively difficult to increase in size, and is thus considered particularly advantageous for mass production.
Also, in the reaction process, volatile oxidants composed of hydrogen peroxide, oxygen, halogens such as bromine and chlorine, halogen oxides such as bleaching powder and hypochlorous acid, or volatile acids composed of oxalic acid, hydrochloric acid, etc. In the presence of this reaction accelerator, the reaction between the compound that liberates phosphate ions in the solution and the metallic iron is promoted, and the reaction may be completed in a short time. However, when oxygen or an oxidizing agent is added, there is a risk of ignition or the like, so it is preferable to pay attention to explosion-proof and gas-phase explosion-proof limit composition management.
Here, the addition of the oxidizing agent not only reacts and dissolves metal iron with a compound that liberates phosphate ions in the solution (usually, it dissolves as divalent iron ions), but further generates trivalent iron ions. There is an effect of changing to iron ions, and it is possible to prevent the residual divalent iron in the ferric phosphate positive electrode material produced in the subsequent firing process. Moreover, acidic reaction promoters, such as hydrochloric acid and oxalic acid, have the effect of accelerating the hydrogen evolution dissolution reaction of iron. Furthermore, it is also effective to add a reaction accelerator that is volatile and has both oxidizing power and acidity, such as nitric acid. In addition, since these oxidizing agents and reaction accelerators are removed in the firing process, there is no possibility of remaining in the positive electrode material.
By baking the reaction product produced by the reaction between the compound that liberates phosphate ions in the solution and metallic iron, ferric phosphate as a positive electrode material is produced. Firing can be performed by selecting firing conditions such as an appropriate temperature range and treatment time in a firing process up to 100 to 900 ° C. as generally employed. This firing is preferably performed in an oxidizing atmosphere in the presence of oxygen such as air in order to promote iron oxidation.
Firing is, for example, a one-step increase from room temperature to firing completion temperature (for example, about 100 to 900 ° C., preferably about 500 to 700 ° C., more preferably about 650 ° C. from the viewpoint of removing moisture in the positive electrode material). This can be done during the temperature and holding process.
When firing from a raw material at a low temperature of about 100 to 500 ° C. (hereinafter referred to as “low temperature firing”), most of the ferric phosphate is amorphous at a firing temperature up to 500 ° C. On the other hand, when firing at a high temperature of about 600 ° C. to 900 ° C. (hereinafter referred to as “high temperature firing”), most of the ferric phosphate has a crystal point group P 321 structure. The range where the firing temperature is 500 to 600 ° C. (hereinafter referred to as “medium temperature firing”) is a temperature range in which the amorphous state shifts to the crystal point group P 321 structure. When the ratio decreases and ferric phosphate of the crystal point group P 321 structure gradually increases and the firing temperature exceeds 600 ° C., the crystal point group P 321 structure becomes dominant as described above.
Amorphous ferric phosphate obtained by low temperature firing, ferric phosphate of crystal point group P 321 structure obtained by high temperature firing, and amorphous and crystal point group P 321 structure obtained by intermediate temperature firing When the electrochemical characteristics of ferric phosphate mixed with the above are compared, it is confirmed that all of them exhibit relatively good discharge characteristics as shown in Example 4 described later. In low-temperature firing and medium-temperature firing ferric phosphate, despite the presence of amorphous, it shows a trajectory that approximates the discharge curve of high-temperature firing. It is suggested that it has a structure similar to the P 321 structure.
Therefore, in the present invention, any of low temperature firing, intermediate temperature firing or high temperature firing may be selected, or the purpose may be selected in a low intermediate temperature (for example, 100 to 600 ° C.) or medium to high temperature (for example, 500 to 900 ° C.). The firing temperature according to the ferric phosphate to be performed can be set. When applied to a non-aqueous electrolyte battery such as a lithium battery later, it is desirable that no moisture remains in the positive electrode material, and high-temperature firing is preferable from the viewpoint of completely removing moisture.
The firing is not limited to one-step firing, but a firing process in a low temperature region (temperature range from room temperature to 300 to 400 ° C .; hereinafter sometimes referred to as “temporary firing”) and firing in a high temperature region. Process [room temperature to firing completion temperature (about 500 to 800 ° C., preferably about 500 to 700 ° C., more preferably about 650 ° C.); hereinafter, sometimes referred to as “main firing”. ] Can be performed in two stages. In such a case, it is preferable to pre-fire a reaction product of a compound that liberates phosphate ions and metallic iron in the solution, and maintain the firing precursor in the above temperature range for about 5 to 24 hours in the main firing. . In addition, the reaction product can be dried and pulverized, if necessary, before calcination, and the calcination precursor can be pulverized and crushed, if necessary, before the main calcination. In addition, the term “sintering” may be referred to simply as including one-step firing, preliminary firing, and main firing.
As described above, according to the method of the present invention, a complicated filtration step is unnecessary in the synthesis of the calcined precursor (ferric phosphate hydrate), and impurities may remain after the raw material is calcined. It is possible to reliably synthesize a positive electrode material that contains almost no impurities. The raw material is a primary raw material or a similar raw material, and is easy to handle and inexpensive, and is suitable for mass production.
In addition, conductive carbon is mixed with the positive electrode material obtained as described above, and carbon obtained by coating the surface of the positive electrode material particles with carbon by performing pulverization and mixing for about 12 hours to 36 hours using, for example, a ball mill or the like. A composite positive electrode material can be obtained. Examples of the conductive carbon include carbon black typified by acetylene black.
By combining carbon with the FePO 4 positive electrode material, as shown in Example 5 described later, the discharge capacity is greatly improved as compared with the case where carbon is not combined. That is, it becomes possible to improve the surface conductivity of the FePO 4 positive electrode material as the positive electrode active material by carbon composite, and the utilization rate of the positive electrode active material is remarkably improved. Therefore, carbon composite is effective for improving the performance of a lithium battery using FePO 4 as a positive electrode material if appropriately applied.
<Lithium battery>
Examples of the lithium battery using the positive electrode material according to the present invention obtained as described above include secondary batteries such as a metal lithium battery, a lithium ion battery, and a lithium polymer battery. The lithium battery of the present invention is also used as a primary battery that only discharges.
Here, the basic configuration of the metal lithium battery using metal lithium as the negative electrode material will be described as an example. The metal lithium battery is a secondary battery characterized in that lithium ions reciprocate between the positive electrode and the negative electrode due to dissolution of lithium metal in the electrolyte and deposition on the negative electrode as the battery is charged and discharged.
As a negative electrode material, in addition to metallic lithium used in metallic lithium batteries, lithium-containing alloys such as lithium aluminum alloys, lithium titanium composite oxides (Li [Li 4/3 Ti 5/3 O 4 ], etc.), lithium transitions A compound made of metal composite nitride (Li 7 MnN 4 , Li 3 FeN 2, etc.), etc., containing lithium from the initial state and having a central element in a reduced state can be used.
As an electrolyte, for example, a mixture of a cyclic organic solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, and γ-butyrolactone as a solvent and a chain organic solvent such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and dimethoxyethane. Liquid electrolytes obtained by dissolving electrolyte substances such as LiPF 6 , LiBF 4 , LiClO 4 , LiCF 3 SO 3 , LiN (CF 3 SO 2 ) 2 , LiC (CF 3 SO 2 ) 3 as solutes in a solvent, and these liquids Gel polymer electrolytes in which electrolytes exist together with polymer gel materials such as polyethylene oxide, polypropylene oxide, polyacrylonitrile, polyvinylidene fluoride, and crosslinked polymer electrolytes obtained by chemically crosslinking these gel polymer electrolytes. It is needed. When the negative electrode material is metallic lithium, a gel polymer electrolyte or a crosslinked polymer electrolyte capable of suppressing the growth of dendrites deposited during charging is preferable. When the electrolyte is a liquid electrolyte, it is possible to isolate the positive electrode and the negative electrode by placing a separator made of polyolefin such as polyethylene / polypropylene between them in order to prevent a short circuit between the positive electrode and the negative electrode. preferable.
The positive electrode and the negative electrode have a conductivity-imparting agent such as carbon black in the positive electrode material and the negative electrode material as long as the effects thereof are not impaired, and fluorine-based polymers such as polytetrafluoroethylene and polyvinylidene fluoride, polyimide, and polyolefin. After mixing and kneading a mixture to which a binder such as a polar organic liquid is added if necessary, a sheet formed into a sheet shape can be used. And a battery is comprised by collecting electricity with a metal night or a metal net. When metallic lithium is used for the negative electrode, a change of Li (0) / Li + occurs due to charge / discharge, and a battery is formed.
The secondary battery of the present invention produced by the above method is a secondary battery that can exhibit high-performance battery characteristics because the electrochemical characteristics of the positive electrode material are excellent. In particular, when used for a metal lithium battery using metal lithium for the negative electrode, good battery characteristics can be obtained.
EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not restrict | limited by these.

（１）正極材料の調製
正極材料（ＦｅＰＯ４）を以下の手順で合成した。
鉄粉６ｇ（和光純薬工業株式会社製；１５０μｍアンダー、純度８５％以上）、リン酸１２．３８５ｇ（和光純薬工業株式会社製；８５％水溶液）、水５０ｃｃからなる原料混合物をボールミル（回転数は２００ｒｐｍ）を用いて１日混合・擂潰した後、９５℃で１日乾燥することにより焼成前駆体を生成した。得られた焼成前駆体を粉砕後、アルミナ製るつぼに入れ、空気存在下において５５０〜７７５℃で８時間焼成を行った。６５０℃で焼成して得た正極材料は、図１に示すＸ線回析結果より結晶点群Ｐ３２１三方晶構造である単一相のリン酸第二鉄（ＦｅＰＯ４）であると同定された。なお、不純物に係る回析ピークは確認されなかった。
（２）２次電池の調製および充放電特性
前記正極材料と、導電性付与剤としてのアセチレンブラック［デンカブラック（登録商標）；電気化学工業株式会社製、５０％プレス品］と、結着剤としてのＰＴＦＥ（ポリテトラフルオロエチレン）を重量比７０：２５：５となるように調整し、まず正極材料とアセチレンブラックをボールミル（回転数は２００ｒｐｍ）を用いて１日混合・擂潰した後に残りのＰＴＦＥと共に混合・混練して、厚さ０．７ｍｍのシート状に成形し、これを直径１ｃｍ（面積０．７８５４ｃｍ２）に打ち抜いてペレット化したものを正極とした。
ステンレス製コイン型電池ケース（ＣＲ２０３２）に金属チタン網、金属ニッケル綱をそれぞれ正負極集電体としてスポット溶接し、前記正極および金属リチウム負極を多孔質ポリエチレン製セパレータ（セルガード社製；セルガード３５０１）を介して組み入れ、電解液としてジメチルカーボネート／エチレンカーボネートの１：１混合溶媒にＬｉＰＦ６を溶解させた１Ｍ溶液を適量注入して封入し、コイン型リチウム２次電池を作製した。なお、一連の電池組み立て作業は、乾燥アルゴンで置換したグローブボックス内で行った。
正極材料を組み込んだ２次電池に対して、正極ペレットの見かけ面積当たり電流密度０．１２７ｍＡ／ｃｍ２および０．５ｍＡ／ｃｍ２において、＋２．０Ｖ〜＋４．５Ｖの作動電圧範囲で充放電を繰り返し行った。なお、放電／充電切換時には、約１時間の開路状態を設けた。
電流密度０．１２７ｍＡ／ｃｍ２での１サイクル目の放電−充電特性は図２に示すとおりであり、初期放電容量は約１３２ｍＡｈ／ｇであった。充放電特性曲線は、２次電池用正極材料として既知であるオリビン型リン酸鉄（ＩＩ）リチウム（ＬｉＦｅＰＯ４）と異なり、平坦な曲線でないことが判った。
また、同電流密度でのサイクル放電−充電特性は図３に示すとおりであり、サイクル回数の増加に伴い放電容量が減少し、底打ち放電容量は約９０ｍＡｈ／ｇであった。
また、電流密度０．５ｍＡ／ｃｍ２での１〜３サイクルの放電−充電特性は図４に示すとおりであり、初期放電容量は約１１２ｍＡｈ／ｇであった。３サイクル目まで次第に容量が低下し、底打ち放電容量は約７８ｍＡｈ／ｇであった。
なお、５５０℃焼成品、６５０℃焼成品、７７５℃焼成品を比較すると、６５０℃焼成品が最も大きい放電容量を示した。(1) Preparation of positive electrode material A positive electrode material (FePO 4 ) was synthesized by the following procedure.
A ball mill (rotating) consists of 6 g of iron powder (Wako Pure Chemical Industries, Ltd .; 150 μm under, purity of 85% or more), 12.385 g of phosphoric acid (85% aqueous solution), and 50 cc of water. The number was mixed and crushed for 1 day using 200 rpm), and then dried at 95 ° C. for 1 day to produce a calcined precursor. The obtained calcined precursor was pulverized and then placed in an alumina crucible and calcined at 550 to 775 ° C. for 8 hours in the presence of air. The positive electrode material obtained by firing at 650 ° C. was identified as single-phase ferric phosphate (FePO 4 ) having a crystal point group P 321 trigonal structure from the X-ray diffraction results shown in FIG. It was. In addition, the diffraction peak which concerns on an impurity was not confirmed.
(2) Preparation and charge / discharge characteristics of secondary battery The positive electrode material, acetylene black [DENKA BLACK (registered trademark); manufactured by Denki Kagaku Kogyo Co., Ltd., 50% pressed product] as a conductivity-imparting agent, and a binder PTFE (polytetrafluoroethylene) as a weight ratio is adjusted to 70: 25: 5. First, the positive electrode material and acetylene black are mixed and crushed for 1 day using a ball mill (rotation speed: 200 rpm) These were mixed and kneaded together with PTFE to form a sheet having a thickness of 0.7 mm, which was punched into a diameter of 1 cm (area 0.7854 cm 2 ) and pelletized to form a positive electrode.
A stainless steel coin-type battery case (CR2032) is spot welded with a metal titanium net and a metal nickel rope as positive and negative current collectors, respectively, and the positive electrode and the metal lithium negative electrode are made of a porous polyethylene separator (manufactured by Cellguard; Cellguard 3501). An appropriate amount of a 1M solution of LiPF 6 dissolved in a 1: 1 mixed solvent of dimethyl carbonate / ethylene carbonate was injected and sealed as an electrolytic solution, and a coin-type lithium secondary battery was produced. In addition, a series of battery assembly operations were performed in a glove box substituted with dry argon.
To the secondary battery incorporating a positive electrode material, the current per apparent area of the cathode pellet density 0.127mA / cm 2 and 0.5 mA / cm 2, charging and discharging at an operating voltage range of + 2.0V to + 4.5V Repeatedly. At the time of discharging / charging switching, an open circuit state of about 1 hour was provided.
The discharge-charge characteristics of the first cycle at a current density of 0.127 mA / cm 2 are as shown in FIG. 2, and the initial discharge capacity was about 132 mAh / g. Unlike the olivine type lithium iron (II) phosphate (LiFePO 4 ) which is known as a positive electrode material for a secondary battery, the charge / discharge characteristic curve was not a flat curve.
Further, the cycle discharge-charge characteristics at the same current density are as shown in FIG. 3, and the discharge capacity decreased with an increase in the number of cycles, and the bottom discharge capacity was about 90 mAh / g.
Further, the discharge-charge characteristics of 1 to 3 cycles at a current density of 0.5 mA / cm 2 are as shown in FIG. 4, and the initial discharge capacity was about 112 mAh / g. The capacity gradually decreased until the third cycle, and the bottom discharge capacity was about 78 mAh / g.
In addition, when the 550 degreeC baked product, the 650 degreeC baked product, and the 775 degreeC baked product were compared, the 650 degreeC baked product showed the largest discharge capacity.

正極材料の調製
正極材料（ＦｅＰＯ４）を以下の手順で合成した。
鉄粉３ｇ（和光純薬工業株式会社製；１５０μｍアンダー、純度８５％以上）、リン酸二水素アンモニウム６．１７９４ｇ（和光純薬工業株式会社製）、水５０ｃｃからなる原料混合物をボールミル（回転数は２００ｒｐｍ）を用いて１日混合・擂潰した後、１００℃で１日乾燥することにより焼成前駆体を生成した。得られた焼成前駆体を粉砕後、アルミナ製るつぼに入れ、空気存在下において６５０℃で１日焼成を行った。これにより合成された正極材料は、図５に示すＸ線回析結果より結晶点群Ｐ３２１三方晶構造である単一相のリン酸第二鉄（ＦｅＰＯ４）であると同定された。なお、不純物に係る回析ピークは確認されなかった。Preparation of positive electrode material A positive electrode material (FePO 4 ) was synthesized by the following procedure.
A raw material mixture consisting of 3 g of iron powder (manufactured by Wako Pure Chemical Industries, Ltd .; under 150 μm, purity of 85% or more), 6.1794 g of ammonium dihydrogen phosphate (manufactured by Wako Pure Chemical Industries, Ltd.), and 50 cc of water is ball milled (rotation speed) Was mixed and crushed for 1 day using 200 rpm, and dried at 100 ° C. for 1 day to produce a calcined precursor. The obtained calcined precursor was pulverized and placed in an alumina crucible and calcined at 650 ° C. for 1 day in the presence of air. The positive electrode material thus synthesized was identified as single-phase ferric phosphate (FePO 4 ) having a crystal point group P 321 trigonal structure from the X-ray diffraction results shown in FIG. In addition, the diffraction peak which concerns on an impurity was not confirmed.

正極材料の調製
正極材料（ＦｅＰＯ４）を以下の手順で合成した。
鉄粉１１ｇ（和光純薬工業株式会社製；１５０μｍアンダー、純度８５％以上）、五酸化リン１３．９７９ｇ（和光純薬工業株式会社製）、水２００ｃｃからなる原料混合物をボールミル（回転数は２００ｒｐｍ）を用いて１日粉砕・混合した後、１００℃で１日乾燥することにより焼成前駆体を生成した。得られた焼成前駆体を粉砕後、アルミナ製るつぼに入れ、空気存在下において６５０℃で１日焼成を行った。これにより合成された正極材料は、図６に示すＸ線回析結果より結晶点群Ｐ３２１三方晶構造である単一相のリン酸第二鉄（ＦｅＰＯ４）であると同定された。なお、不純物に係る回析ピークは確認されなかった。Preparation of positive electrode material A positive electrode material (FePO 4 ) was synthesized by the following procedure.
A raw material mixture consisting of 11 g of iron powder (manufactured by Wako Pure Chemical Industries, Ltd .; under 150 μm, purity 85% or more), 13.979 g of phosphorus pentoxide (manufactured by Wako Pure Chemical Industries, Ltd.), and 200 cc of water is ball milled (rotation speed is 200 rpm) ) For 1 day, and then dried at 100 ° C. for 1 day to produce a calcined precursor. The obtained calcined precursor was pulverized and placed in an alumina crucible and calcined at 650 ° C. for 1 day in the presence of air. The positive electrode material thus synthesized was identified as single-phase ferric phosphate (FePO 4 ) having a crystal point group P 321 trigonal structure from the X-ray diffraction results shown in FIG. In addition, the diffraction peak which concerns on an impurity was not confirmed.

正極材料の調製
正極材料（ＦｅＰＯ４）を以下の手順で合成した。
化学量論比の鉄粉（和光純薬工業株式会社製；１５０μｍアンダー、純度８５％以上）１１．１６９ｇと五酸化リン（和光純薬工業株式会社製）１４．４８３ｇに純水２００ｍｌを加え、遊星ボールミルを用いて回転数２００ｒｐｍにて１日粉砕・混合した。内容物を取り出して乾燥後、１００℃、２００℃、３５０℃、５００℃および６５０℃の温度にて大気中で１２時間焼成した。得られたそれぞれの正極材料をめのう乳鉢で粉砕した後、実施例１と同様にして、正極を調製し、金属リチウム負極を用いてコイン型リチウム２次電池を作製した。
合成されたそれぞれの正極材料のＸ線回折結果を図７に示す。図７より、焼成温度１００℃から５００℃未満の試料は回折ピークを持たない非晶質であり、５００℃焼成試料は大部分が非晶質でごく僅かに三方晶Ｐ３２１の結晶部分を有していた。これに対し、実施例３と同様の温度である６５０℃での焼成試料は、Ｐ３２ １の結晶構造を有することが判る。
それぞれの正極材料を組込んだコイン型リチウム２次電池の第一サイクルにおける放電、および充電の特性を図８に示す。測定条件は、２５℃、見かけ面積当たり電流密度０．２ｍＡ／ｃｍ２において、２Ｖ〜４Ｖの折返し充放電とした。これらの正極材料において、結晶質および非晶質試料間の充放電プロファイル上の差異はほとんど認められず、その放電電圧プロファイルは、充放電反応中に酸化態・還元態の２相が共存するオリビン型結晶構造（斜方晶Ｐｎｍａ）のリン酸鉄リチウムが示す平坦なものとは明らかに異なり、均一相反応の場合に見られる単調減少の曲線となった。
本実施例では、鉄粉と５酸化リンという安価な出発原料を用い、１００〜６５０℃の合成温度において充放電測定を行った結果、３５０℃以上の焼成試料において、従来の報告（前掲）の容量値４０ｍＡｈ／ｇを大きく上回る最高１１５ｍＡｈ／ｇという容量値を示した（図８参照）。また、１００℃という極めて低い焼成温度で得られた正極材料においても、１００ｍＡｈ／ｇを超える放電容量を示すことが注目される。
さらに、放電後のコイン型リチウム２次電池からこれらの正極材料を取り出して測定したＸ線回折結果は、それぞれの製造時のものと大差がなく（結果は図示せず）、充放電によって新たな相の生成は認められなかった。このことより、本実施例で得られた正極材料は充放電中においていずれも安定であることが確認できた。
また、各正極材料について、ＴＧ（熱重量変化）測定を行ったところ、加熱脱水による重量減少は２００℃以上の焼成温度の正極材料ではほとんど認められなかった。しかし、フーリエ赤外吸光分析では、結晶水の存在に起因する１６００ｃｍ−１のＨ−Ｏ−Ｈ変角モードの吸収が完全に消滅していたのは、６５０℃焼成の結晶質試料のみであった。長期間に渡ってリチウム電池の性能を安定に維持するためには、電池内部に水分が存在しないことが好ましいことから、長期的性能の観点からは、６５０℃程度の焼成条件が有利であると考えられる。Preparation of positive electrode material A positive electrode material (FePO 4 ) was synthesized by the following procedure.
Stoichiometric iron powder (Wako Pure Chemical Industries, Ltd .; 150 μm under, purity of 85% or more) 11.169 g and phosphorus pentoxide (Wako Pure Chemical Industries, Ltd.) 14.483 g are added with 200 ml of pure water, Using a planetary ball mill, the mixture was pulverized and mixed for 1 day at 200 rpm. After the contents were taken out and dried, they were baked in air at temperatures of 100 ° C., 200 ° C., 350 ° C., 500 ° C. and 650 ° C. for 12 hours. After each obtained positive electrode material was pulverized in an agate mortar, a positive electrode was prepared in the same manner as in Example 1, and a coin-type lithium secondary battery was produced using a metal lithium negative electrode.
The X-ray diffraction result of each synthesized positive electrode material is shown in FIG. From FIG. 7, the sample with a firing temperature of 100 ° C. to less than 500 ° C. is amorphous with no diffraction peak, and the 500 ° C. fired sample is mostly amorphous and has a very slight crystal part of trigonal P 321. Was. On the other hand, it can be seen that the fired sample at 650 ° C., which is the same temperature as in Example 3, has a crystal structure of P 32 1 .
FIG. 8 shows the discharge and charge characteristics in the first cycle of the coin-type lithium secondary battery incorporating each positive electrode material. The measurement conditions were 2 V to 4 V folded charge / discharge at 25 ° C. and a current density per apparent area of 0.2 mA / cm 2 . In these positive electrode materials, there is almost no difference in charge and discharge profiles between crystalline and amorphous samples, and the discharge voltage profile is olivine in which two phases of oxidized and reduced states coexist during the charge and discharge reaction. A flat crystal structure (orthorhombic Pnma) of lithium iron phosphate was clearly different from the flat one, and it was a monotonically decreasing curve seen in the case of homogeneous phase reaction.
In this example, as a result of performing charge / discharge measurement at a synthesis temperature of 100 to 650 ° C. using inexpensive starting materials such as iron powder and phosphorus pentoxide, in a fired sample of 350 ° C. or higher, the conventional report (supra). The maximum capacitance value was 115 mAh / g, which greatly exceeded the capacitance value of 40 mAh / g (see FIG. 8). It is also noted that a positive electrode material obtained at an extremely low firing temperature of 100 ° C. exhibits a discharge capacity exceeding 100 mAh / g.
Furthermore, the X-ray diffraction results obtained by measuring these positive electrode materials from the coin-type lithium secondary battery after discharge are not significantly different from those at the time of manufacture (results are not shown), and new results are obtained by charging and discharging. No phase formation was observed. From this, it has confirmed that all the positive electrode materials obtained by the present Example were stable during charging / discharging.
Moreover, when TG (thermogravimetric change) measurement was performed for each positive electrode material, weight loss due to heat dehydration was hardly observed in the positive electrode material having a firing temperature of 200 ° C. or higher. However, in the Fourier infrared absorption analysis, the absorption of the 1600 cm −1 H—O—H bending mode due to the presence of crystal water completely disappeared only in the crystalline sample fired at 650 ° C. It was. In order to stably maintain the performance of the lithium battery over a long period of time, it is preferable that no water is present inside the battery, so that firing conditions of about 650 ° C. are advantageous from the viewpoint of long-term performance. Conceivable.

実施例４で合成した６５０℃焼成のＰ３２１結晶構造を有する正極材料ＦｅＰＯ４に対し、アセチレンブラック（電気化学工業株式会社製、５０％プレス品）を２５重量％となるように加えた後、遊星ボールミルを用いて２００ｒｐｍにて１日粉砕混合を行い、アセチレンブラックを正極材料粒子の表面に複合化させた正極材料（以下、「炭素複合化正極材料」を記す）を作成した。この正極材料から、実施例１と同様にして、正極を調製し、金属リチウム負極を用いてコイン型リチウム２次電池を作製した。
このコイン型リチウム２次電池について充放電試験を行った。その結果を図９に示す。測定条件は、２５℃、見かけ面積当たり電流密度０．２ｍＡ／ｃｍ２において、２．６Ｖ〜４Ｖの折返し充放電とした。なお、図９には、炭素複合化を行わない６５０℃正極材料の測定結果も併記した（但し、２．０Ｖ〜４Ｖの折返し充放電条件とした）。
図９から、ＦｅＰＯ４正極材料に炭素を複合化することにより、炭素を複合化しない場合に比べて放電容量が大幅に改善され、約１３０ｍＡｈ／ｇもの値を示すことが判る。これは、正極活物質としてのＦｅＰＯ４正極材料の表面導電性が炭素複合化によって改善され、正極活物質の利用率が格段に向上したためであると考えられる。以上のことから、炭素複合化は、ＦｅＰＯ４を正極材料として用いるリチウム電池の性能向上に有効であることが示された。After adding acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., 50% pressed product) to 25 wt% to the positive electrode material FePO 4 having a P 321 crystal structure sintered at 650 ° C. synthesized in Example 4, Using a planetary ball mill, the mixture was pulverized and mixed at 200 rpm for 1 day to prepare a positive electrode material (hereinafter referred to as “carbon composite positive electrode material”) in which acetylene black was combined on the surface of the positive electrode material particles. A positive electrode was prepared from this positive electrode material in the same manner as in Example 1, and a coin-type lithium secondary battery was prepared using a metal lithium negative electrode.
A charge / discharge test was performed on the coin-type lithium secondary battery. The result is shown in FIG. The measurement conditions were 2.6 V to 4 V folded charge / discharge at 25 ° C. and a current density of 0.2 mA / cm 2 per apparent area. In addition, in FIG. 9, the measurement result of the 650 degreeC positive electrode material which does not carbon-composite was also written together (however, it was set as the reversal charge / discharge conditions of 2.0V-4V).
From FIG. 9, it can be seen that by combining carbon with the FePO 4 positive electrode material, the discharge capacity is significantly improved as compared with the case where carbon is not combined, and the value is about 130 mAh / g. This is considered to be because the surface conductivity of the FePO 4 positive electrode material as the positive electrode active material was improved by the carbon composite, and the utilization factor of the positive electrode active material was remarkably improved. From the above, it was shown that carbon composite is effective in improving the performance of a lithium battery using FePO 4 as a positive electrode material.

正極材料の調製
正極材料（ＦｅＰＯ４）を以下の手順で合成した。
化学量論比の鉄粉（和光純薬工業株式会社製；１５０μｍアンダー、純度８５％以上）１１．１６９ｇと五酸化リン（和光純薬工業株式会社製）１４．４８３ｇに純水２００ｍｌを加え、水冷冷却管を取付けたガラス製三角フラスコに入れ、ホットスターラーで１００℃に加熱しながら３日間還流した（以下、本実施例における製法を「還流法」と記す）。内容物を取り出して乾燥後、６５０℃の温度にて大気中で２４時間焼成した。得られた正極材料をめのう乳鉢で粉砕した後、実施例１と同様にして正極を調製し、金属リチウム負極を用いてコイン型リチウム２次電池を作製した。
合成された正極材料のＸ線回折結果を図１０に示す。図１０より、還流法によっても、遊星ボールミルで原料を粉砕・反応させた実施例４における６５０℃焼成試料と同様にＰ３２１の結晶構造を有するＦｅＰＯ４が得られることが判る。
この正極材料を組込んだコイン型リチウム２次電池について、実施例４と同様の条件で充放電試験を実施したところ、図８の上段に示した６５０℃焼成試料と同様の放電電圧プロファイルおよび約１１５ｍＡｈ／ｇの放電容量が得られた（測定結果の図示は省略する）。
以上のことから、還流法によっても、原料を遊星ボールミル等で擂潰して溶解、反応させた場合と同等の焼成前駆体を合成可能であること、および、この焼成前駆体を焼成することによって、高性能の正極材料が得られることが確認された。Preparation of positive electrode material A positive electrode material (FePO 4 ) was synthesized by the following procedure.
Stoichiometric iron powder (Wako Pure Chemical Industries, Ltd .; 150 μm under, purity of 85% or more) 11.169 g and phosphorus pentoxide (Wako Pure Chemical Industries, Ltd.) 14.483 g are added with 200 ml of pure water, The flask was placed in a glass Erlenmeyer flask equipped with a water-cooled condenser, and refluxed for 3 days while being heated to 100 ° C. with a hot stirrer (hereinafter, the production method in this example is referred to as “reflux method”). The contents were taken out and dried, followed by baking at a temperature of 650 ° C. for 24 hours. After pulverizing the obtained positive electrode material in an agate mortar, a positive electrode was prepared in the same manner as in Example 1, and a coin-type lithium secondary battery was produced using a metal lithium negative electrode.
The X-ray diffraction result of the synthesized positive electrode material is shown in FIG. From FIG. 10, it can be seen that FePO 4 having a crystal structure of P 321 can also be obtained by the reflux method, similarly to the 650 ° C. fired sample in Example 4 in which the raw material was pulverized and reacted with a planetary ball mill.
When a charge / discharge test was performed on the coin-type lithium secondary battery incorporating this positive electrode material under the same conditions as in Example 4, a discharge voltage profile similar to that of the 650 ° C. firing sample shown in the upper part of FIG. A discharge capacity of 115 mAh / g was obtained (the measurement results are not shown).
From the above, even by the reflux method, it is possible to synthesize the firing precursor equivalent to the case where the raw material is crushed and dissolved by a planetary ball mill or the like, and by firing this firing precursor, It was confirmed that a high-performance positive electrode material was obtained.

本発明方法によれば、確実に、かつ、容易に２次電池用正極材料としてのリン酸第二鉄（ＦｅＰＯ４）を製造することができる。本発明方法によって製造された正極材料は、例えば金属リチウム電池における正極材料として好適に用いることができる。According to the method of the present invention, ferric phosphate (FePO 4 ) as a positive electrode material for a secondary battery can be reliably and easily produced. The positive electrode material produced by the method of the present invention can be suitably used as a positive electrode material in, for example, a metal lithium battery.

Claims (5)

溶液中でリン酸イオンを遊離する化合物と、金属鉄とを混合し、前記金属鉄を溶解させて反応させた後、焼成を行うことによりリン酸第二鉄を合成することを特徴とする、リチウム電池用正極材料の製造方法。A compound that liberates phosphate ions in a solution and metallic iron are mixed, the metallic iron is dissolved and reacted, and then ferric phosphate is synthesized by firing. A method for producing a positive electrode material for a lithium battery. 溶液中でリン酸イオンを遊離する化合物と、金属鉄とを水溶液中で擂潰しながら反応させた後、焼成を行うことによりリン酸第二鉄を合成することを特徴とする、リチウム電池用正極材料の製造方法。A lithium battery positive electrode characterized in that ferric phosphate is synthesized by reacting a compound that liberates phosphate ions in a solution and metallic iron in an aqueous solution while being crushed and then firing. Material manufacturing method. 請求項１または請求項２において、前記溶液中でリン酸イオンを遊離する化合物が、リン酸、五酸化リン、またはリン酸二水素アンモニウムであることを特徴とする、リチウム電池用正極材料の製造方法。3. The positive electrode material for a lithium battery according to claim 1, wherein the compound that liberates phosphate ions in the solution is phosphoric acid, phosphorus pentoxide, or ammonium dihydrogen phosphate. Method. 請求項１から請求項３のいずれか１項に記載の方法により製造された正極材料に導電性炭素を混合し、粉砕混合することを特徴とする、リチウム電池用正極材料の製造方法。A method for producing a positive electrode material for a lithium battery, comprising mixing a conductive carbon with the positive electrode material produced by the method according to any one of claims 1 to 3 and pulverizing and mixing the mixture. 請求項１から請求項４のいずれか１項に記載の方法により製造された正極材料を構成要素とすることを特徴とする、リチウム電池。A lithium battery comprising a positive electrode material produced by the method according to claim 1 as a constituent element.
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